1. Field of the Invention
This invention relates to light emitting diodes (LEDs) and more particularly to electronic circuitry for providing power to LEDs from alternating current sources.
2. Description of Related Art
The many potential advantages of LEDs as a light source, compared to incandescent bulbs, may soon make LEDs viable as replacements for most applications currently using incandescents. LEDs are being developed with increasing amounts of light output—some currently available can provide about the same amount of illumination as a conventional 60 watt light bulb. LEDs are also relatively efficient with recent LED prototypes being demonstrated with a white light efficiency of 131 lumens per watt, while consuming only 20 mA of power.
Moreover, improvements have been made in the color of the light produced by LEDs, and LEDs also have great advantages over incandescent bulbs with respect to longevity, since some LEDs can last for approximately 100,000 hours. The current installed cost for LED lighting, however, is much greater, although the cost of LED lighting will no doubt eventually be competitive with incandescent lighting.
Another issue with LED lighting concerns the electrical circuitry necessary to power an LED. Unlike an incandescent bulb which operates equally well regardless of the polarity of the electric current applied to it, an LED generates light only when a voltage source is applied with a positive electrical polarity, i.e., when the LED is forward biased. Since an LED is a diode, the voltage applied across the LED is logarithmically related to the current. Hence the power can be considered generally proportional to the current, and the power supply for an LED should thus be a current source, to keep the power constant.
For low power applications requiring only a small number of LEDs, such as in mobile telephones, for example, the necessary direct current (DC) power can be readily supplied by a battery source. The need for a direct current (DC) source can cause problems, however, for applications requiring high amounts of light output, such as lighting for buildings.
One current practice is to use a linear or switching power supply to convert alternating current (AC) power to DC constant current. Such power supplies, however, require bulky components, such as transformers, inductors and capacitors. This can eliminate LEDs as a lighting choice where space is limited. In addition, the complexity of the conversion from AC to DC inherently introduces power losses.
Moreover, if the DC power is transmitted over a distance, system complexity and cost is typically increased. For these reasons, it is desirable to drive LED lighting directly from an AC source of power. Among other desirable features of AC is that the voltage can be readily stepped up or down using a transformer. An AC powered LED, however, will operate only during the positive voltage portion of the power cycle, causing the LED to turn on and off with the frequency of the AC supply.